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Tfap2a promotes specification and maturation of neurons in the inner ear through modulation of Bmp, Fgf and notch signaling.

Kantarci H, Edlund RK, Groves AK, Riley BB - PLoS Genet. (2015)

Bottom Line: Directly blocking either Fgf or Notch caused less dramatic acceleration of SAG development without neuronal death, whereas blocking both pathways mimicked all observed effects of Tfap2a overexpression, including apoptosis of mature neurons.Blocking Bmp signaling reversed the effects of overexpressing Tfap2a.Together, these data support a model in which Tfap2a, acting through Bmp7a, modulates Fgf and Notch signaling to control the duration, amount and speed of SAG neural development.

View Article: PubMed Central - PubMed

Affiliation: Biology Department, Texas A&M University, College Station, Texas, United States of America.

ABSTRACT
Neurons of the statoacoustic ganglion (SAG) transmit auditory and vestibular information from the inner ear to the hindbrain. SAG neuroblasts originate in the floor of the otic vesicle. New neuroblasts soon delaminate and migrate towards the hindbrain while continuing to proliferate, a phase known as transit amplification. SAG cells eventually come to rest between the ear and hindbrain before terminally differentiating. Regulation of these events is only partially understood. Fgf initiates neuroblast specification within the ear. Subsequently, Fgf secreted by mature SAG neurons exceeds a maximum threshold, serving to terminate specification and delay maturation of transit-amplifying cells. Notch signaling also limits SAG development, but how it is coordinated with Fgf is unknown. Here we show that transcription factor Tfap2a coordinates multiple signaling pathways to promote neurogenesis in the zebrafish inner ear. In both zebrafish and chick, Tfap2a is expressed in a ventrolateral domain of the otic vesicle that includes neurogenic precursors. Functional studies were conducted in zebrafish. Loss of Tfap2a elevated Fgf and Notch signaling, thereby inhibiting SAG specification and slowing maturation of transit-amplifying cells. Conversely, overexpression of Tfap2a inhibited Fgf and Notch signaling, leading to excess and accelerated SAG production. However, most SAG neurons produced by Tfap2a overexpression died soon after maturation. Directly blocking either Fgf or Notch caused less dramatic acceleration of SAG development without neuronal death, whereas blocking both pathways mimicked all observed effects of Tfap2a overexpression, including apoptosis of mature neurons. Analysis of genetic mosaics showed that Tfap2a acts non-autonomously to inhibit Fgf. This led to the discovery that Tfap2a activates expression of Bmp7a, which in turn inhibits both Fgf and Notch signaling. Blocking Bmp signaling reversed the effects of overexpressing Tfap2a. Together, these data support a model in which Tfap2a, acting through Bmp7a, modulates Fgf and Notch signaling to control the duration, amount and speed of SAG neural development.

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Tfap2a regulates the level of Fgf and Notch Signaling in the otic vesicle.(A-V) Whole-mount images (dorsal up, anterior left) showing dorsolateral views of the otic vesicle (outlined). (A-R) Expression of the indicated genes in wild-type embryos, hs:tfap2a embryos and tfap2a-/- mutants (A-I) or tfap2a morphants (J-R) at 26 hpf (A-L, P-R) and 28 (M-O) hpf. (S, T) Cross-sections (dorsal up, medial left) passing through the utricular macula show spry4 expression at 28 hpf in a control embryo and tfap2a-/- mutant. (U-X) Whole-mounts showing expression of etv5b at 26 hpf. Activation of hs:tfap2a diminishes etv5b expression (U, V), activation of hs:fgf8 leads to global upregulation of etv5b (W), and co-activation of hs:fgf8 and hs:tfap2a restores etv5b to near normal (X). (Y) Cross-sections (dorsal up, medial left) passing just posterior to the utricular macula showing ngn1 at 24 hpf following a 35°C heat shock at 23 hpf. Reduction in the ngn1 domain caused by knockdown of tfap2a is rescued by weak activation of hs:dnfgfr1. (Z) Mean and standard deviation of the total number of ngn1 positive cells in the otic epithelium at 24 hpf for the genotypes and knockdowns indicated in the color key (counted from serial sections, n = 3–6 ears per time point). Asterisks (*) indicate statistically significant differences between the groups indicated in brackets.
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pgen.1005037.g006: Tfap2a regulates the level of Fgf and Notch Signaling in the otic vesicle.(A-V) Whole-mount images (dorsal up, anterior left) showing dorsolateral views of the otic vesicle (outlined). (A-R) Expression of the indicated genes in wild-type embryos, hs:tfap2a embryos and tfap2a-/- mutants (A-I) or tfap2a morphants (J-R) at 26 hpf (A-L, P-R) and 28 (M-O) hpf. (S, T) Cross-sections (dorsal up, medial left) passing through the utricular macula show spry4 expression at 28 hpf in a control embryo and tfap2a-/- mutant. (U-X) Whole-mounts showing expression of etv5b at 26 hpf. Activation of hs:tfap2a diminishes etv5b expression (U, V), activation of hs:fgf8 leads to global upregulation of etv5b (W), and co-activation of hs:fgf8 and hs:tfap2a restores etv5b to near normal (X). (Y) Cross-sections (dorsal up, medial left) passing just posterior to the utricular macula showing ngn1 at 24 hpf following a 35°C heat shock at 23 hpf. Reduction in the ngn1 domain caused by knockdown of tfap2a is rescued by weak activation of hs:dnfgfr1. (Z) Mean and standard deviation of the total number of ngn1 positive cells in the otic epithelium at 24 hpf for the genotypes and knockdowns indicated in the color key (counted from serial sections, n = 3–6 ears per time point). Asterisks (*) indicate statistically significant differences between the groups indicated in brackets.

Mentions: We next investigated whether Tfap2a activity influences Notch and Fgf signaling, pathways known to regulate development of SAG neurons. For example, Delta-Notch signaling is normally activated by neurogenic factors Ngn1 and Neurod and serves as a feedback inhibitor of neurogenesis [2,33,34]. Disruption of Delta-Notch signaling leads to excess neural specification and precocious differentiation [35], similar to the effects of misexpression of tfap2a. Here we observed that expression of deltaA and deltaB was increased in tfap2a-/- mutants and, conversely, delta gene expression was strongly impaired following overexpression of tfap2a (Fig. 6A-F). Similar changes were observed for the Notch target gene her4, which increased in tfap2a-/- mutants and decreased following activation of hs:tfap2a (Fig. 6G-I). Thus Tfap2a appears to inhibit Notch activity during development of SAG neurons by inhibiting expression of Notch ligands.


Tfap2a promotes specification and maturation of neurons in the inner ear through modulation of Bmp, Fgf and notch signaling.

Kantarci H, Edlund RK, Groves AK, Riley BB - PLoS Genet. (2015)

Tfap2a regulates the level of Fgf and Notch Signaling in the otic vesicle.(A-V) Whole-mount images (dorsal up, anterior left) showing dorsolateral views of the otic vesicle (outlined). (A-R) Expression of the indicated genes in wild-type embryos, hs:tfap2a embryos and tfap2a-/- mutants (A-I) or tfap2a morphants (J-R) at 26 hpf (A-L, P-R) and 28 (M-O) hpf. (S, T) Cross-sections (dorsal up, medial left) passing through the utricular macula show spry4 expression at 28 hpf in a control embryo and tfap2a-/- mutant. (U-X) Whole-mounts showing expression of etv5b at 26 hpf. Activation of hs:tfap2a diminishes etv5b expression (U, V), activation of hs:fgf8 leads to global upregulation of etv5b (W), and co-activation of hs:fgf8 and hs:tfap2a restores etv5b to near normal (X). (Y) Cross-sections (dorsal up, medial left) passing just posterior to the utricular macula showing ngn1 at 24 hpf following a 35°C heat shock at 23 hpf. Reduction in the ngn1 domain caused by knockdown of tfap2a is rescued by weak activation of hs:dnfgfr1. (Z) Mean and standard deviation of the total number of ngn1 positive cells in the otic epithelium at 24 hpf for the genotypes and knockdowns indicated in the color key (counted from serial sections, n = 3–6 ears per time point). Asterisks (*) indicate statistically significant differences between the groups indicated in brackets.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4364372&req=5

pgen.1005037.g006: Tfap2a regulates the level of Fgf and Notch Signaling in the otic vesicle.(A-V) Whole-mount images (dorsal up, anterior left) showing dorsolateral views of the otic vesicle (outlined). (A-R) Expression of the indicated genes in wild-type embryos, hs:tfap2a embryos and tfap2a-/- mutants (A-I) or tfap2a morphants (J-R) at 26 hpf (A-L, P-R) and 28 (M-O) hpf. (S, T) Cross-sections (dorsal up, medial left) passing through the utricular macula show spry4 expression at 28 hpf in a control embryo and tfap2a-/- mutant. (U-X) Whole-mounts showing expression of etv5b at 26 hpf. Activation of hs:tfap2a diminishes etv5b expression (U, V), activation of hs:fgf8 leads to global upregulation of etv5b (W), and co-activation of hs:fgf8 and hs:tfap2a restores etv5b to near normal (X). (Y) Cross-sections (dorsal up, medial left) passing just posterior to the utricular macula showing ngn1 at 24 hpf following a 35°C heat shock at 23 hpf. Reduction in the ngn1 domain caused by knockdown of tfap2a is rescued by weak activation of hs:dnfgfr1. (Z) Mean and standard deviation of the total number of ngn1 positive cells in the otic epithelium at 24 hpf for the genotypes and knockdowns indicated in the color key (counted from serial sections, n = 3–6 ears per time point). Asterisks (*) indicate statistically significant differences between the groups indicated in brackets.
Mentions: We next investigated whether Tfap2a activity influences Notch and Fgf signaling, pathways known to regulate development of SAG neurons. For example, Delta-Notch signaling is normally activated by neurogenic factors Ngn1 and Neurod and serves as a feedback inhibitor of neurogenesis [2,33,34]. Disruption of Delta-Notch signaling leads to excess neural specification and precocious differentiation [35], similar to the effects of misexpression of tfap2a. Here we observed that expression of deltaA and deltaB was increased in tfap2a-/- mutants and, conversely, delta gene expression was strongly impaired following overexpression of tfap2a (Fig. 6A-F). Similar changes were observed for the Notch target gene her4, which increased in tfap2a-/- mutants and decreased following activation of hs:tfap2a (Fig. 6G-I). Thus Tfap2a appears to inhibit Notch activity during development of SAG neurons by inhibiting expression of Notch ligands.

Bottom Line: Directly blocking either Fgf or Notch caused less dramatic acceleration of SAG development without neuronal death, whereas blocking both pathways mimicked all observed effects of Tfap2a overexpression, including apoptosis of mature neurons.Blocking Bmp signaling reversed the effects of overexpressing Tfap2a.Together, these data support a model in which Tfap2a, acting through Bmp7a, modulates Fgf and Notch signaling to control the duration, amount and speed of SAG neural development.

View Article: PubMed Central - PubMed

Affiliation: Biology Department, Texas A&M University, College Station, Texas, United States of America.

ABSTRACT
Neurons of the statoacoustic ganglion (SAG) transmit auditory and vestibular information from the inner ear to the hindbrain. SAG neuroblasts originate in the floor of the otic vesicle. New neuroblasts soon delaminate and migrate towards the hindbrain while continuing to proliferate, a phase known as transit amplification. SAG cells eventually come to rest between the ear and hindbrain before terminally differentiating. Regulation of these events is only partially understood. Fgf initiates neuroblast specification within the ear. Subsequently, Fgf secreted by mature SAG neurons exceeds a maximum threshold, serving to terminate specification and delay maturation of transit-amplifying cells. Notch signaling also limits SAG development, but how it is coordinated with Fgf is unknown. Here we show that transcription factor Tfap2a coordinates multiple signaling pathways to promote neurogenesis in the zebrafish inner ear. In both zebrafish and chick, Tfap2a is expressed in a ventrolateral domain of the otic vesicle that includes neurogenic precursors. Functional studies were conducted in zebrafish. Loss of Tfap2a elevated Fgf and Notch signaling, thereby inhibiting SAG specification and slowing maturation of transit-amplifying cells. Conversely, overexpression of Tfap2a inhibited Fgf and Notch signaling, leading to excess and accelerated SAG production. However, most SAG neurons produced by Tfap2a overexpression died soon after maturation. Directly blocking either Fgf or Notch caused less dramatic acceleration of SAG development without neuronal death, whereas blocking both pathways mimicked all observed effects of Tfap2a overexpression, including apoptosis of mature neurons. Analysis of genetic mosaics showed that Tfap2a acts non-autonomously to inhibit Fgf. This led to the discovery that Tfap2a activates expression of Bmp7a, which in turn inhibits both Fgf and Notch signaling. Blocking Bmp signaling reversed the effects of overexpressing Tfap2a. Together, these data support a model in which Tfap2a, acting through Bmp7a, modulates Fgf and Notch signaling to control the duration, amount and speed of SAG neural development.

Show MeSH
Related in: MedlinePlus